1. Field of the Invention
The present invention relates to a mobile communication terminal apparatus having a function of requiring, for example, handoff of a base station, a control circuit of the apparatus, and a handoff control method in a cellular mobile communication system.
2. Description of the Related Art
In recent years, a cellular mobile communication system has rapidly spread. In the cellular mobile communication system, a plurality of base stations are scattered/disposed in a service area and these base stations form respective radio zones called cells. Moreover, the mobile communication terminal apparatus is connected to a base station of the radio zone in which the terminal apparatus exists over a radio channel, and the base station is connected to the terminal apparatus of a communication destination via a communication network. This realizes communication among the terminal apparatuses.
In this type of system, when the mobile communication terminal apparatus moves between the radio zones during waiting or communicating, handoff is performed. A handoff operation will be described hereinafter by means of an example of a system in which a code division multiple access (CDMA) method is employed.
The CDMA mobile communication terminal apparatus measures a reception level of a pilot signal transmitted from the base station which is establishing synchronization during waiting or communicating. Moreover, the respective reception levels of the pilot signals transmitted from a plurality of base stations (neighboring base stations) disposed around the base station which is establishing the synchronization are also measured. A strength of the reception level is represented by a ratio (Ec/Io) of a whole reception energy (Io) of the pilot signal after inverse spread to an energy (Ec) of the pilot signal arriving from the base station as a measurement object. Moreover, when the reception level of the pilot signal arriving from the neighboring base station has a strength of a predetermined level or more strength, the following determinations are performed. In one determination, it is determined whether or not a strength (Ec/Io)PNa2 of the reception level of a pilot signal PNa2 arriving from the base station establishing the synchronization is less than a value designated by the base station. In the other determination, it is determined whether or not a strength (Ec/Io)other of the reception level of each pilot signal arriving from a remaining base station is more than another value designated by the base station. Moreover, when answers to these determinations are both affirmative, a handoff request message called a pilot strength measurement message (PSMM) is generated and transmitted to the base station establishing the synchronization. In this handoff request message, the reception level of the pilot signal arriving from the base station which is establishing the synchronization, and the reception levels of the respective pilot signals arriving from the respective neighboring base stations are inserted.
On the other hand, on receiving the handoff request message from the mobile communication terminal apparatus, the base station determines a base station as a handoff destination based on the reception level inserted in the message. Moreover, a notice of a name of the base station as the determined handoff destination is transmitted to the mobile communication terminal apparatus which requires handoff. On receiving the notice, the mobile communication terminal apparatus first returns the response message indicating the acknowledgment to the base station. Subsequently, the base station as a synchronization destination is switched in response to the notice. A handoff processing of the mobile communication terminal apparatus is performed in this manner.
Additionally, this type of conventional system has the following problems to be solved. That is, when the mobile communication terminal apparatus transmits the handoff request message, the apparatus waits for a return of an acknowledgment response message from the base station. Moreover, when the acknowledgment response message is not returned within a given time, a quality of the radio channel is determined to be temporarily deteriorated. Subsequently, the base station repeats transmission of the handoff request message by a predetermined number of times (e.g., nine times at maximum). That is, a retransmission procedure of the handoff request message is executed. FIG. 12 shows one example of the sequence. In FIG. 12, (SEQ1(1)) indicates a first transmission of a sequence number “1”, and (SEQ1(2)) indicates a second transmission of the sequence number “1”.
However, a retransmission time interval in the retransmission procedure is determined by a standard for each system. For example, the interval is set to 400 msec or more in the CDMA system defined by IS-95. Therefore, transmission frequency of the handoff request message by the retransmission procedure is reduced, Therefore, a long period of time is sometimes required from when the retransmitted message reaches the base station until handoff is actually executed.
In general, when the handoff request message is transmitted, the communication state with the base station is generally bad in many cases. In this case, the message cannot easily be transmitted to the base station from the mobile communication terminal apparatus. When the mobile communication terminal apparatus moves at a high speed in the communication state, the mobile communication terminal apparatus moves along a considerably long distance during a period of message retransmission to the base station from the mobile communication terminal apparatus. In this case, the content of the message having reached the base station turns to old information. When the base station determines the handoff destination based on the old information, an inappropriate base station is designated as the handoff destination. Moreover, as a result, handoff failure is disadvantageously generated.